1. Field of the Invention
The present invention provides a method for the production of extremely flat silicon troughs in a silicon substrate for MOS-transistors. The object of the invention is achieved by firstly employing a mask over the silicon substrate, forming the trough by a localized etching process such that each trough is made slightly anisotropic and has a degree of lateral underetching of said mask and then filling rate-controllably each of said troughs with silicon material by a selective epitaxy process.
2. Prior Art
The production of doped silicon troughs in a silicon substrate of the opposite conductivity type and orientation (100) for MOS-transistors is well known in the Art. This production, can, as described in the publication E. Sirtl and H. Seiter "Selective Epitaxy Under Quasi-Equilibrium Conditions," Semiconductor Silicon 1969, pages 189 to 199, in particular for (111)-substrates, be effected by means of selective epitaxy. The epitaxial process has the advantage that a standard doping or doping profile can be produced, which can be achieved only with great difficulty with pure diffusion. In the epitaxial process firstly a masking technique is used to etch troughs into the semiconductor material of the one conductivity type, which are subsequently filled with silicon material of the opposite conductivity type. Following the removal of the masking layers the filled troughs should be as flat as possible, that is, where as possible there should be no graduations at the surface between the material of the substrate and the material of the troughs. The requirement of as ideal a flatness as possible is based on the further process steps. For example during these subsequent process steps, mounds formed at the junction between the substrate material and the material of the full trough could scratch and damage the masks required for the masking technique.
The silicon mounds which were formed at the mask edges during the production of silicon troughs by selective epitaxy in accordance with the previously known processes must be eliminated for the reasons given above, which, in the methods of the prior art, has been effected by grinding or polishing.
A particular disadvantage of the mechanical removal of these mounds consists in the fact that fresh disturbances are caused by polishing damage. For example the materials of the troughs and the substrate are removed at different rates as they exhibit differing hardnesses.